Such technique has been proposed that a surrounding image of driver's own vehicle is acquired by a camera and the acquired image is presented to the driver so as to assist the driver in checking a surrounding situation of the driver's own vehicle and thus contribute to the driver's safe driving. For example, a plurality of cameras are provided around a driver's own vehicle and images which are picked up by respective cameras are synthesized, enabling the driver to visually recognize a wide-ranging image of the surroundings of the driver's own vehicle.
A plurality of cameras have to be installed on predetermined positions and in predetermined camera-directions so as to obtain an image which provides no feeling of strangeness to a driver, that is, a synthesis image in which no seams of images are out of alignment or images are not inclined, as a result of synchronization of a plurality of picked-up images. However, cameras are attached on a vehicle, so that installation positions and angles of the cameras may change due to vibration caused by moving of the vehicle and the like. Therefore, whether or not a gap is generated on installation positions and angles of the cameras has to be confirmed at arbitrary timing after start of use such as shipping and vehicle inspection, and thus calibration processing of the installation positions and angles of the cameras has to be performed.
In the calibration processing of installation positions and angles of cameras, a marker (jig) which is installed on a prescribed position in a camera shooting range is shot and a marker position on the shot image, in other words, a coordinate of a feature point of the marker on a picked-up image is associated with a prescribed position. As a marker used for the calibration processing, a marker which has a pattern of which a feature point is easily extracted on a picked-up image is preferably used. For example, a checkered marker is generally used. Here, in a checkered marker, a central point of an intersection of a checkered pattern is extracted as a feature point, for example.
As a method for extracting a feature point of a checkered marker, such technique has been disclosed that a color boundary line of color regions which constitute a checkered pattern on a picked-up image is detected and an intersecting point is obtained by using the detected color boundary line so as to extract a feature point of a marker, in Japanese Laid-open Patent Publication No. 2010-87743.